Capacitive sensors can be used to measure displacement accurately, by exhibiting a change in capacitance as a function of relative displacement of the two conductive plates that form a capacitance. Typically, the capacitive sensor is constructed using precision metal plates that are in close proximity, while an electric field is maintained between them. In many cases, the resulting variable capacitance is usually relatively small, for example on the order of less than 10 picoFarads but may depend widely on the geometry of the sensor. A measuring circuit is coupled to the plates and produces an output signal that represents a measure of the capacitance. Typical measuring circuits include the use of an analog timer integrated circuit to generate an oscillating signal whose frequency varies as a function of, and is inversely proportional to, the capacitance to be measured. A micro controller can then be used to count pulses, in response to the oscillating signal, within a given period, which translates into the frequency of the oscillating signal and hence the capacitance. The conventional measurement circuit has well known limitations that result in an error in the measured capacitance. The error may be caused by the existence of parasitic capacitance on the input pins of the timer integrated circuit (which are coupled to the capacitance to be measured). The parasitic capacitance erroneously adds to the measured capacitance value. The effects of parasitic capacitance on the input pins of the timer integrated circuit may be compensated for by electronic subtraction using passive or active compensation devices.
Other techniques for measuring a variable capacitance include an operational amplifier (op amp) integrator approach in which the op amp drives a precision current into the capacitor, and determines the capacitance by assessing an integration time. With that approach, the input pin capacitance of the op amp used for integration remains at a virtual ground, thereby relieving any concerns with parasitic capacitance on the input pin.